feat: add strftimedelta pattern

[chebertpinard]

Suggestion for a new pattern type to support passing a date with a timedelta. The syntax would look as the following.

Pattern("/path/to/data/{date:strftimedelta(-6h;%Y%m%d)}{date:strftimedelta(-6h;%H)}_006")

Would allow an easier selection of data at validity time in forecast files.

[FussyDuck]

All committers have signed the CLA.

[sandorkertesz]

@chebertpinard, I added a test for Pattern:

tests/utils/test_patterns.py

Can I ask you to merge the latests develop into your PR and add at least one test to it for your new feature?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.29%. Comparing base (ea24172) to head (2ceac47).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #606   +/-   ##
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  Coverage    90.29%   90.29%           
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  Files          154      154           
  Lines        11572    11572           
  Branches       546      546           
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  Hits         10449    10449           
  Misses         953      953           
  Partials       170      170           

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